DESCRIPTION

The kqueue() system call provides a generic method of notifying the user
when an event happens or a condition holds, based on the results of small
pieces of kernel code termed filters. A kevent is identified by the
(ident, filter) pair; there may only be one unique kevent per kqueue.
The filter is executed upon the initial registration of a kevent in order
to detect whether a preexisting condition is present, and is also
executed whenever an event is passed to the filter for evaluation. If
the filter determines that the condition should be reported, then the
kevent is placed on the kqueue for the user to retrieve.
The filter is also run when the user attempts to retrieve the kevent from
the kqueue. If the filter indicates that the condition that triggered
the event no longer holds, the kevent is removed from the kqueue and is
not returned.
Multiple events which trigger the filter do not result in multiple
kevents being placed on the kqueue; instead, the filter will aggregate
the events into a single struct kevent. Calling close() on a file
descriptor will remove any kevents that reference the descriptor.
The kqueue() system call creates a new kernel event queue and returns a
descriptor. The queue is not inherited by a child created with fork(2).
However, if rfork(2) is called without the RFFDG flag, then the
descriptor table is shared, which will allow sharing of the kqueue
between two processes.
The kevent() system call is used to register events with the queue, and
return any pending events to the user. The changelist argument is a
pointer to an array of kevent structures, as defined in All changes
contained in the changelist are applied before any pending events are
read from the queue. The nchanges argument gives the size of changelist.
The eventlist argument is a pointer to an array of kevent structures.
The nevents argument determines the size of eventlist. When nevents is
zero, kevent() will return immediately even if there is a timeout
specified unlike select(2). If timeout is a non-NULL pointer, it
specifies a maximum interval to wait for an event, which will be
interpreted as a struct timespec. If timeout is a NULL pointer, kevent()
waits indefinitely. To effect a poll, the timeout argument should be
non-NULL, pointing to a zero-valued timespec structure. The same array
may be used for the changelist and eventlist.
The EV_SET() macro is provided for ease of initializing a kevent
structure.
The kevent structure is defined as:
struct kevent {
uintptr_t ident; /* identifier for this event */
short filter; /* filter for event */
u_short flags; /* action flags for kqueue */
u_int fflags; /* filter flag value */
intptr_t data; /* filter data value */
void *udata; /* opaque user data identifier */
};
The fields of structkevent are:
ident Value used to identify this event. The exact interpretation
is determined by the attached filter, but often is a file
descriptor.
filter Identifies the kernel filter used to process this event. The
pre-defined system filters are described below.
flags Actions to perform on the event.
fflags Filter-specific flags.
data Filter-specific data value.
udata Opaque user-defined value passed through the kernel unchanged.
The flags field can contain the following values:
EV_ADD Adds the event to the kqueue. Re-adding an existing event
will modify the parameters of the original event, and not
result in a duplicate entry. Adding an event
automatically enables it, unless overridden by the
EV_DISABLE flag.
EV_ENABLE Permit kevent() to return the event if it is triggered.
EV_DISABLE Disable the event so kevent() will not return it. The
filter itself is not disabled.
EV_DELETE Removes the event from the kqueue. Events which are
attached to file descriptors are automatically deleted on
the last close of the descriptor.
EV_ONESHOT Causes the event to return only the first occurrence of
the filter being triggered. After the user retrieves the
event from the kqueue, it is deleted.
EV_CLEAR After the event is retrieved by the user, its state is
reset. This is useful for filters which report state
transitions instead of the current state. Note that some
filters may automatically set this flag internally.
EV_EOF Filters may set this flag to indicate filter-specific EOF
condition.
EV_ERROR See RETURNVALUES below.
The predefined system filters are listed below. Arguments may be passed
to and from the filter via the fflags and data fields in the kevent
structure.
EVFILT_READ Takes a descriptor as the identifier, and returns whenever
there is data available to read. The behavior of the
filter is slightly different depending on the descriptor
type.
Sockets
Sockets which have previously been passed to listen()
return when there is an incoming connection pending.
data contains the size of the listen backlog.
Other socket descriptors return when there is data to
be read, subject to the SO_RCVLOWAT value of the
socket buffer. This may be overridden with a per-
filter low water mark at the time the filter is added
by setting the NOTE_LOWAT flag in fflags, and
specifying the new low water mark in data. On return,
data contains the number of bytes of protocol data
available to read.
If the read direction of the socket has shutdown, then
the filter also sets EV_EOF in flags, and returns the
socket error (if any) in fflags. It is possible for
EOF to be returned (indicating the connection is gone)
while there is still data pending in the socket
buffer.
Vnodes
Returns when the file pointer is not at the end of
file. data contains the offset from current position
to end of file, and may be negative.
Fifos, Pipes
Returns when the there is data to read; data contains
the number of bytes available.
When the last writer disconnects, the filter will set
EV_EOF in flags. This may be cleared by passing in
EV_CLEAR, at which point the filter will resume
waiting for data to become available before returning.
BPF devices
Returns when the BPF buffer is full, the BPF timeout
has expired, or when the BPF has “immediate mode”
enabled and there is any data to read; data contains
the number of bytes available.
EVFILT_WRITE Takes a descriptor as the identifier, and returns whenever
it is possible to write to the descriptor. For sockets,
pipes and fifos, data will contain the amount of space
remaining in the write buffer. The filter will set EV_EOF
when the reader disconnects, and for the fifo case, this
may be cleared by use of EV_CLEAR. Note that this filter
is not supported for vnodes or BPF devices.
For sockets, the low water mark and socket error handling
is identical to the EVFILT_READ case.
EVFILT_AIO The sigevent portion of the AIO request is filled in, with
sigev_notify_kqueue containing the descriptor of the
kqueue that the event should be attached to, sigev_value
containing the udata value, and sigev_notify set to
SIGEV_KEVENT. When the aio_*() system call is made, the
event will be registered with the specified kqueue, and
the ident argument set to the structaiocb returned by the
aio_*() system call. The filter returns under the same
conditions as aio_error.
EVFILT_VNODE Takes a file descriptor as the identifier and the events
to watch for in fflags, and returns when one or more of
the requested events occurs on the descriptor. The events
to monitor are:
NOTE_DELETE The unlink() system call was called on the
file referenced by the descriptor.
NOTE_WRITE A write occurred on the file referenced by
the descriptor.
NOTE_EXTEND The file referenced by the descriptor was
extended.
NOTE_ATTRIB The file referenced by the descriptor had
its attributes changed.
NOTE_LINK The link count on the file changed.
NOTE_RENAME The file referenced by the descriptor was
renamed.
NOTE_REVOKE Access to the file was revoked via
revoke(2) or the underlying file system was
unmounted.
On return, fflags contains the events which triggered the
filter.
EVFILT_PROC Takes the process ID to monitor as the identifier and the
events to watch for in fflags, and returns when the
process performs one or more of the requested events. If
a process can normally see another process, it can attach
an event to it. The events to monitor are:
NOTE_EXIT The process has exited.
NOTE_FORK The process has called fork().
NOTE_EXEC The process has executed a new process
via execve(2) or similar call.
NOTE_TRACK Follow a process across fork() calls.
The parent process will return with
NOTE_TRACK set in the fflags field, while
the child process will return with
NOTE_CHILD set in fflags and the parent
PID in data.
NOTE_TRACKERR This flag is returned if the system was
unable to attach an event to the child
process, usually due to resource
limitations.
On return, fflags contains the events which triggered the
filter.
EVFILT_SIGNAL Takes the signal number to monitor as the identifier and
returns when the given signal is delivered to the process.
This coexists with the signal() and sigaction()
facilities, and has a lower precedence. The filter will
record all attempts to deliver a signal to a process, even
if the signal has been marked as SIG_IGN. Event
notification happens after normal signal delivery
processing. data returns the number of times the signal
has occurred since the last call to kevent(). This filter
automatically sets the EV_CLEAR flag internally.
EVFILT_TIMER Establishes an arbitrary timer identified by ident. When
adding a timer, data specifies the timeout period in
milliseconds. The timer will be periodic unless
EV_ONESHOT is specified. On return, data contains the
number of times the timeout has expired since the last
call to kevent(). This filter automatically sets the
EV_CLEAR flag internally. There is a system wide limit on
the number of timers which is controlled by the
kern.kq_calloutmax sysctl.
EVFILT_NETDEV Takes a descriptor to a network interface as the
identifier, and the events to watch for in fflags. It
returns, when one or more of the requested events occur on
the descriptor. The events to monitor are:
NOTE_LINKUP The link is up.
NOTE_LINKDOWN The link is down.
NOTE_LINKINV The link state is invalid.
On return, fflags contains the events which triggered the
filter.

RETURNVALUES

The kqueue() system call creates a new kernel event queue and returns a
file descriptor. If there was an error creating the kernel event queue,
a value of -1 is returned and errno set.
The kevent() system call returns the number of events placed in the
eventlist, up to the value given by nevents. If an error occurs while
processing an element of the changelist and there is enough room in the
eventlist, then the event will be placed in the eventlist with EV_ERROR
set in flags and the system error in data. Otherwise, -1 will be
returned, and errno will be set to indicate the error condition. If the
time limit expires, then kevent() returns 0.

ERRORS

The kqueue() system call fails if:
[ENOMEM] The kernel failed to allocate enough memory for the
kernel queue.
[EMFILE] The per-process descriptor table is full.
[ENFILE] The system file table is full.
The kevent() system call fails if:
[EACCES] The process does not have permission to register a
filter.
[EFAULT] There was an error reading or writing the kevent
structure.
[EBADF] The specified descriptor is invalid.
[EINTR] A signal was delivered before the timeout expired and
before any events were placed on the kqueue for
return.
[EINVAL] The specified time limit or filter is invalid.
[ENOENT] The event could not be found to be modified or
deleted.
[ENOMEM] No memory was available to register the event or, in
the special case of a timer, the maximum number of
timers has been exceeded. This maximum is
configurable via the kern.kq_calloutmax sysctl.
[ESRCH] The specified process to attach to does not exist.

SEEALSO

HISTORY

The kqueue() and kevent() system calls first appeared in FreeBSD 4.1.

AUTHORS

The kqueue() system and this manual page were written by Jonathan Lemon
〈jlemon@FreeBSD.org〉.

BUGS

It is currently not possible to watch a vnode(9) that resides on anything
but a UFS file system.
The EVFILT_NETDEV filter is currently only implemented for devices that
use the miibus(4) driver for LINKUP and LINKDOWN operations. Therefore,
it will not work with many non-ethernet devices.
The timeout value is limited to 24 hours; longer timeouts will be
silently reinterpreted as 24 hours.